Spatial Light Modulators (SLMs) have found numerous applications in the areas of optical information processing, projection displays, video and graphics monitors, televisions, and electrophotographic printing. SLMs are devices that modulate incident light in a spatial pattern to form a light image corresponding to an electrical or optical input. The incident light may be modulated in its phase, intensity, polarization, or direction. The light modulation may be achieved by a variety of materials exhibiting various electro-optic or magneto-optic effects, and by materials that modulate light by surface deformation.
A SLM is typically comprised of an area or linear array of addressable picture elements (pixels). Source pixel data is first formatted by an associated control circuit, usually external to the SLM, and then loaded into the pixel array one frame at a time. This pixel data may be written to the pixel array using a variety of algorithms, i.e. sequentially top-to-bottom one pixel line at a time, interleaving by sequentially addressing top-to-bottom every other pixel line, such as the odd rows of pixels, and then returning to address the even pixel lines, etc. In cathode ray tubes (CRTs), this data writing technique is known as rasterizing, whereby a high powered electron gun scans across the pixel elements of a phosphor screen left to right, one line at a time. This pixel address data writing scheme is equally applicable to liquid crystal displays (LCDs) as well.
A recent innovation of Texas Instruments Incorporated of Dallas Tex., is the digital micromirror device or the deformable mirror device (collectively DMD). The DMD is revolutionary in that it is truly a digital display device and an integrated circuit solution. The DMD is an electro/mechanical/optical SLM suitable for use in displays, projectors and hardcopy printers. The DMD is a monolithic single-chip integrated circuit SLM, comprised of a high density array of 16 micron square movable micromirrors on 17 micron centers. These mirrors are fabricated over address circuitry including an array of SRAM cells and address electrodes. Each mirror forms one pixel of the DMD array and is bistable, that is to say, stable in one of two positions, wherein a source of light directed upon the mirror array will be reflected in one of two directions. In one stable "on" mirror position, incident light to that mirror will be reflected to a projector lens and focused on a display screen or a photosensitive element of a printer. In the other "off" mirror position, light directed on the mirror will be deflected to a light absorber. Each mirror of the array is individually controlled to either direct incident light into the projector lens, or to the light absorber. The projector lens ultimately focuses and magnifies the modulated light from the pixel mirrors onto a display screen and produce an image in the case of a display. If each pixel mirror of the DMD array is in the "on" position, the displayed image will be an array of bright pixels.
For a more detailed discussion of the DMD device and uses, cross reference is made to U.S. Pat. No. 5,061,049 to Hornbeck, entitled "Spatial Light Modulator and Method"; U.S. Pat. No. 5,079,544 to DeMond, et al, entitled "Standard Independent Digitized Video System"; and U.S. Pat. No. 5,105,369 to Nelson, entitled "Printing System Exposure Module Alignment Method and Apparatus of Manufacture", each patent being assigned to the same assignee of the present invention and the teachings of each are incorporated herein by reference. Gray scale of the pixels forming the image is achieved by pulse-width modulation techniques of the mirrors, such as that described in U.S. Pat. No. 5,278,652, entitled "DMD Architecture and Timing for Use in a Pulse-Width Modulated Display System", assigned to the same assignee of the present invention, and the teachings of which are incorporated herein by reference.
As detailed in commonly assigned U.S. Pat. No. 5,535,047 entitled "Active Yoke Hidden Hinge Digital Micromirror Device", and shown in FIG. 1 of the present application, there is disclosed a digital micromirror device (DMD) spatial light modulator shown at 10. DMD 10 is a single-chip integrated circuit seen to include an array of micromirrors 30 monolithically fabricated over a memory cell array formed upon the substrate. Each pixel mirror 30 is seen to include a square mirror supported upon and elevated above a butterfly shaped yoke generally shown at 32 by a rectangular support post 34. Support post 34 extends downward from the center of the mirror 30, and is attached to the center of the yoke 32 along a torsion axis thereof, as shown, to balance the center of mass of mirror 30 upon yoke 32. Yoke 32 is axially supported along a central axis thereof by a pair of torsion hinges 40. The other end of each torsion hinge 40 is attached to and supported by a hinge support post cap 42 defined on top of a respective hinge support post 44. A pair of elevated mirror address electrodes 50 and 54 are supported by a respective address support post. The address support posts, and the hinge support posts 44, support the address electrodes 50 and 54, the torsion hinges 40, and the yoke 32 away from and above a bias/reset bus 60, and a pair of substrate level address electrode pads 26 and 28. When mirror 30 and yoke 32 are together rotated about the torsion axis of the yoke, defined by the hinges 40, a pair of yoke tips 58 on the side of the yoke 32 that is deflected land upon and engage the bias/reset bus 60 at a landing site 62. For more detailed discussion of this conventional DMD, the teachings of commonly assigned U.S. Pat. No. 5,535,047 are incorporated herein by reference.
Still referring to FIG. 1, it can be seen that the support post 34 extends downward from the reflective modulation surface of the square mirror 30 and defines support post edges at 64. These support post edges 64 conventionally have dimensions of 3.times.4 .mu.m, which edges form a rectangle and are oriented either perpendicular or parallel to the incoming beam of light. Referring to FIG. 2, there is shown the light diffracted back into the projection optics when all mirrors are in the off-state. It can be seen that these support post edges 64 diffract the incident light into the projection system optics when the mirrors 30 are tilted to the off state, the diffraction seen as light dots 66, thus reducing the contrast ratio of the formed display image. Also seen in FIG. 2 is diffracted light from the underlying superstructure and address circuitry from between the mirror edges.
There is desired a DMD spatial light modulator forming an image having an improved contrast ratio whereby the diffraction of incident light from the support post edges 64 into the projection optics is substantially reduced.